The Coordination Chemistry of FeCl₃ and FeCl₂ to Bis[2‐(2,3‐dihydroxyphenyl)‐6‐pyridylmethyl](2‐pyridylmethyl)amine: Access to a Diiron(III) Compound with an Unusual Pentagonal‐Bipyramidal/Square‐Pyramidal Environment

Abstract

Coordination of FeCl3 to the title ligand yields a mononuclear iron(III) complex 1, which was characterized by spectroscopic techniques and X-ray diffraction. The ligand is (kappa3-N) tridentate and the metal, which lies in a pseudo-octahedral environment, is bound to a phenolate group from the catechol substituent. The dichloroiron(II) complex 2 was easily obtained by metalation of the ligand with FeCl2 and characterized by various spectroscopic techniques. In their cyclic voltammograms both 1 and 2 display the same reversible FeII/FeIII wave at E1/2=10 mV (vs. SCE). Reduction of compound 1 with Zn/Hg yields 2', which displays identical properties to 2. Taken together, these findings indicate that in spite of the different oxidation state of the metal in 2, no major geometrical/structural change is observed at the metal center with respect to 1. The reaction of 2 with dioxygen in the absence of organic substrates proceeds extremely rapidly and yields compound 3, which is a diiron(III) derivative whose X-ray crystal structure is also reported. The possibility of a radical-based mechanism is discussed. Compound 3 displays an unusual geometry: one iron(III) center is seven-coordinate, whereas the other lies in a square-pyramidal environment. The two iron atoms are bridged by the catecholato substituents. To the best of our knowledge, 3 is the first example of a seven-coordinate iron(III) derivative with tris(2-pyridylmethyl)amine ligands.

Reactivity of Molecular Dioxygen towards a Series of Isostructural Dichloroiron(III) Complexes with Tripodal Tetraamine Ligands: General Access to μ‐Oxodiiron(III) Complexes and Effect of α‐Fluorination on the Reaction Kinetics

“…These values lie in the expected range for neutral electrolytes and thus indicate that dissociation must be minor at these concentrations: by comparison, L values close to 100 and 150 S cm 2 mol À1 were found for pure mono-and dicationic species, respectively. [24,31,34] All data are listed in Table 3. FeCl 2 is qualitatively identical to that already reported for the brominated analogue BrTPAFeCl 2 , and assignment can easily be made by direct comparison.…”

“…[24] Indeed, in the present study, zinc amalgam was also found to be an easy way to recover the starting material quantitatively for complexes of L 0 and L…”

“…Very often, the presence of non-innocent ligands such as (a-ceto) carboxylates, thiolates, or catecholates proved to be crucial. [18][19][20][21][22][23][24] In some cases, examples of formation of m-oxo dinuclear complexes were reported, and a mechanism that parallels what is described in porphyrin chemistry was discussed. [20,21,23] Recently, Fe II complexes with amide-containing macrocycles were shown to react with dioxygen.…”

Coordination and Bioinorganic Chemistry of Aryl-Appended Tris(2-Pyridylmethyl)amine Ligands

“…Aryl-appended TPA ligands have been used in the preparation of coordination complexes of Mn(II), [22,28] Fe(II) and Fe(III), [20,[23][24][25]28,34,35] Co(II), [22,28,31] Ni(II), [22,28,31,36] Cu(I) and Cu(II), [21,27,28,[37][38][39] and Zn(II). [22,28,31] Mn(II).…”

“…[22,23] The latter reaction has thus far only been used for the preparation of 6-(4-OMePh)TPA, 6-(3-CNPh)TPA, BCATTPA, and a-MePh 2 TPA ( Figure 1). [24][25][26] A procedure for the preparation of o-d 1 -6-phenyl-2-pyridinecarboxaldehyde has also been reported. [23] To construct an aryl-appended tris(2-pyridylmethyl)amine (TPA) ligand starting from the aryl-appended carboxaldehyde, a common synthetic pathway involves (1) reduction of the aldehyde group using NaCNBH 3 [27] or NaBH 4 [28] to generate a primary alcohol, (2) treatment of this alcohol with SOCl 2 [21,22,27] or PBr 3 [28] to generate a 2-halomethyl intermediate, and (3) heating of this intermediate with a 2-(aminomethyl)pyridine derivative in the presence of base to produce a mono-, di-or triaryl-appended chelate ligand.…”